public Token EvaluateExpression()
{
int index = 0;
while (expressionParts.Count > 1)
{
PartExpression leftPart = expressionParts[index];
PartExpression rightPart = expressionParts[index + 1];
//wrong order of operations
if ((leftPart.Op == '+' || leftPart.Op == '-') && (rightPart.Op == '*' || rightPart.Op == '/'))
{
index++;
continue;
}
//correct order
else
{
PartExpression combinedPart = CombineParts(leftPart, rightPart);
expressionParts.RemoveAt(index); //removes leftPart
expressionParts.RemoveAt(index); //removes rightPart
expressionParts.Insert(index, combinedPart);
index = 0; //reset to start of expression after each combination
}
}
//after while loop there will only be one part left
PartExpression lastPart = expressionParts[0];
double resultNum = lastPart.Value;
Token resultToken = new Token("num", resultNum);
return(resultToken);
}
//combine two part expressions by preforming the mathematical operation between them
PartExpression CombineParts(PartExpression leftPart, PartExpression rightPart)
{
double combinedValue;
switch (leftPart.Op)
{
case '+':
combinedValue = leftPart.Value + rightPart.Value;
break;
case '-':
combinedValue = leftPart.Value - rightPart.Value;
break;
case '*':
combinedValue = leftPart.Value * rightPart.Value;
break;
case '/':
combinedValue = leftPart.Value / rightPart.Value;
break;
default:
throw new Exception("Unexpected Operator - Syntax error.");
}
Token combinedValueToken = new Token("num", combinedValue);
//move the operator from the right part to the newly combined part
Token rightOp = new Token("op", rightPart.Op);
return(new PartExpression(combinedValueToken, rightOp));
}
void BuildExpressionParts(List <Token> tokens)
{
for (int i = 0; i < tokens.Count; i++)
{
Token next;
if (i + 1 >= tokens.Count) //last token in list
{
next = null;
}
else
{
next = tokens[i + 1];
}
if (next != null)
{
PartExpression part = new PartExpression(tokens[i], next);
expressionParts.Add(part);
tokens.RemoveAt(i + 1); //removes operator token
}
else
{
PartExpression part = new PartExpression(tokens[i]);
expressionParts.Add(part);
}
}
}
